Roundcube works fine wirh ISPConfig 3 as any webmail client with pop3 or imap support odes. Just create a nwe website e.g. webmail.yourdomain.com and you can install the normal roundcube version from the rouncube website in this website and it will work with ispconfig.

1) Downloaded roundcubemail-0.2.1.tar.gz - Stable release from http://roundcube.net/downloads
2) Downloaded your "ISPConfig3_for_0.2.1_with" file
3) Uploaded roundcubemail-0.2.1.tar.gz, and "un" tar xvfz roundcubemail-0.2.1.tar.gz
4) Unziped your ISPConfig3_for_0.2.1_with, and uploaded the files in the directorys to the corresponding ones from roundcubemail
5) Renamed the db.inc.php.dist to db.inc.php and main.inc.php.dist to main.inc.php
6) changed the line $rcmail_config .... (in db.inc.php) to $rcmail_config['ispconfig_db'] = 'mysql://ispconfigassword@localhost/dbispconfig';
(note that as password in the above line I used the one found in /usr/local/ispconfig/interface/lib/config.inc.php, and that the password is (md5?) encrypted)

Now when accessing my domain/roundcubemail I get a DATABASE ERROR: CONNECTION FAILED!

Okay got it sort of working.
I did do the install as mentioned in the install, but I replaced the "$rcmail_config['db_dsnw'] = 'mysql://roundcubeass@localhost/roundcubemail';" with the one from you!
I should have read your install.txt better as it is saying " Edit the file {Place to roundcubefolder}/config/db.inc.php and add before ?>"

Anyway. I needed to remove the .htaccess file as it was giving my server a hard time with "ERROR 500 - Internal Server Error!".
Did you also needed to remove the .htaccess file to make RoundCube work on ISPconfig3?

An other two things that do not work for me are the Autoreply (it will give me a 500 error when trying to access the page) and changing the password.
I do see the password page, but when I hit "save" I get (like the Autoreply) a 500 error, and the password does also not get changed.

I got it all working!
For some reason the password that I got the 1st time from "/usr/local/ispconfig/interface/lib/config.inc.php" did not match at all in what I used in "/config/db.inc.php"
I'm 100% sure that I did put in the correct password, but again it did not match at all when I had a new look today!

I did do an apt-get update and apt-get upgrade today, but this could not have caused this problem!

I have already the solution to make the installation easier. The solution is the remote framework, but until it isn't finished, I get forced to use my own way to change the settings. So that users don't have to wait.